The present invention relates to a method of manufacturing a reinforcing element used for asbestos free friction materials for brakes, clutches, etc, and provides particularly the method permitting said reinforcing element to be manufactured with excellent performance and stabilized quality and in low cost.
Conventionally, the woven friction materials have been manufactured generally in such a way that the asbestos is used as a reinforcing element. Thermosetting resin such as phenol resin, rubber compounding material, etc, are adhered to said element and, after preforming, it is heated under pressure in a metal mold for thermoforming.
Recently, because of the environmental problem against the human body, the demand for asbestos free friction materials has increased. For manufacturing the reinforcing element for these asbestos free friction materials, glass fibers are used generally as main components and, to the twisted yarn of said glass fibers with organic fibers, inorganic fibers, metallic fibers or the like, thermosetting resin, rubber compounding material, etc. are adhered. After preforming, it is heated under pressure in a metal mold for thermoforming similarly to above.
However, in order to make the frictional performance of the friction materials using artificial fibers such as said glass fibers etc. for a reinforcing element which is equal to or better than those having friction material using asbestos for the reinforcing element, it is necessary to adhere more rubber compounding material to the reinforcing element in more stabilized state and still the dispersibility of fibers should also be better than in the case of asbestos as a reinforcing element.
Therefore, for the adherence of more amounts of rubber compounding material etc. to the twisted yarn of glass fibers etc., a method has been adopted traditionally that the glass fibers are submitted to the bulking treatment, or that the twisted yarns with other fibers low in the bulk density are used.
In the form of twisted yarns as described above, however, the following inconveniences occur because of the twisting of heterogeneous yarns with same length.
(1) Due to the twisting tension, the degree of bulkiness of bulky roving is lowered. PA1 (2) By twisting, the degree of bulkiness is lowered resulting in a decrease in the dispersibility of fibers after forming. PA1 (3) When adhering and permeating resin, rubber compounding material, etc. to the twisted yarn, the degree of bulkiness is lowered due to the tension acting on the twisted yarn. PA1 (4) Due to the fluctuation of tension acting on the twisted yarn, the degree of bulkiness fluctuates.
Consequently, when using the twisted yarns as described above, the adhering rate of rubber compounding material etc. is lowered, the fluctuation of adhering rate becomes high, further the dispersibility of fibers is lowered, and the fluctuation of dispersibility becomes high resulting in the difficulty in obtaining the stabilized frictional performance after forming of the friction materials, which effects pose the problems.